Power metal-oxide-silicon field-effect transistors (MOSFET) are employed in applications requiring high voltages and high currents. One type of Power MOSFETs uses a trench gate structure where the transistor gate is provided in a vertical trench formed at the surface of the substrate. The trench gate is isolated from the substrate by a gate oxide layer lining the sidewall and the base of the trench. The source and body regions are formed adjacent the trench at the surface of the substrate and the drain region is formed on the opposite surface of the substrate. As thus configured, the channel of the transistor is formed in body region along the vertical sidewall of the trench. Power MOSFETs using a trench gate are sometimes referred to as trench MOSFETs, or trench gate power MOSFETs, or trench-gated vertical power MOSFET.
In some applications, trench gate power MOSFET devices benefit from using a dual oxide thickness trench gate structure. In a dual oxide thickness trench gate structure, the trench gate is formed in a trench lined with a liner oxide layer at a bottom portion of the trench that is thicker than the thin gate oxide layer at the top portion of the trench. FIG. 1 duplicates FIG. 3N of commonly assigned U.S. patent application Ser. No. 13/776,523 and illustrates an example of a dual oxide thickness trench gate structure for a trench gate power MOSFET device. U.S. patent application Ser. No. 13/776,523 is entitled “Termination Trench For Power MOSFET Applications,” filed Feb. 25, 2013, and is incorporated herein by reference in its entirety. Conventional fabrication processes for forming the dual oxide thickness trench gate structure involve forming a trench in the silicon substrate to a first depth, forming a nitride spacer on the sidewalls of the trench, and then etching the silicon substrate in the trench further to a second depth using the nitride spacer as a self-aligned mask. Then, the conventional processes perform thermal oxidation to grow the liner oxide layer at the bottom portion of the trench. The nitride spacer is removed and thermal oxidation is performed to grow the gate oxide layer at the top portion of the trench.
Challenges exist in forming the dual oxide thickness trench gate structure. For example, particle debris may become lodged in the bottom of the trench during the second trench etch process. The particle debris inhibits the liner oxidation and causes shorts between the trench gate and the silicon substrate.